The exchange of oxygen and carbon dioxide between small non-capillary blood vessels and tissues will influence the distribution of the gas tensions within the microcirculation. In particular, this exchange will effect the intralumenal gas tensions of arterial and venous vessels and the tension in the tissue being perfused. The objective of this proposal study is to design and solve a model of this gas exchange, and then test this model by experiments with canine gracilis muscle. The experimental preparation will involve measurement, by blood samples, of arterial and venous PO2 and PCO2; and, by needle electrodes, the spatially averaged tissue PO2 and PCO2 during muscle rest and at exercise. A set of experiments will specifically measure the variability of PO2 and PCO2 in skeletal muscle recorded by needle electrodes. Subsequently, predictions of the theoretical model will be compared to the experimental results. In addition, to examine a consequence of non-capillary exchange, the relationship of mean tissue gas tensions and vascular resistance will be measured. A better understanding of the importance of this small vessel exchange will follow from this theoretical model and the rigorous statistical analysis of the experimental results.